In ophthalmology it has been established, in case of defective vision, to form the cornea of the human eye with its approximate thickness of 500 μm through ablation of tissue in order to correct myopia, hypermetropia, and astigmatism. This is called refractive surgery. Nowadays it is executed either with mechanical means, such as microkeratomes in combination with laser radiation, or purely optically with laser radiation. Thereby, the laser radiation of the ArF excimer laser, the pulsed radiation of which exhibits a wavelength of 193 nm, has proven successful. With radiation of said wavelength, satisfactory ablation results with minimal, negligible side effects are achieved.
Thereto, two different methods for executing such an operation are known. With photorefractive keratectomy (PRK), the upper epithelium layer with an approximate thickness of 50 μm is irreversibly removed from Bowman's membrane with a surgical instrument called a hockey knife and the laser ablation is executed on the stromal surface. Within a healing process, a new epithelium forms on the laser-treated surface after surgery. However, this is associated with pain for the patient.
During laser-supported intrastromal keratomileusis (LASIK), a stromal flap with an approximate thickness of 160 μm is detached from the cornea by means of a mechanical microkeratome and folded back around a non-detached area similar to a hinge. Thereby, the flap is usually produced concentrically with the pupil. The laser treatment is executed in the intrastromal tissue. After treatment, the flap is folded back.
Thereby, the patients experience minimal pain after surgery and quick vision recovery. However, the procedure with a microkeratome is fraught with risk, and the remaining thickness of the cornea, available for the refractive correction, is less than with PRK.
Recently, strongly focused radiation from femtosecond lasers has been applied in order to execute incisions in the cornea (Femto-LASIK). Such devices are also called laser microkeratome. Thereby, a photodisruption is produced in the focus, which leads to a minimal formation of bubbles in the stromal tissue. If focal spot is set next to focal spot by means of a scanner system, random incisions (perforations) can be made in the cornea. Said incisions are hereinafter called laser incisions. For example, from US 2006/0155265 A1 (Intralase Corp.) it is known to cut the flap by means of a femtosecond laser system. The ablation of the stromal tissue, necessary for a refractive correction, is subsequently executed conservatively by means of an excimer laser, completely forgoing a mechanical treatment; however, two laser systems are required.
In WO 2008/064771 A1 (Carl Zeiss Meditec AG), a femtosecond laser system is described, which can also prepare the flap but is additionally capable of separating the ablation of stromal tissue, necessary for a refractive correction, through dual incisions for the preparation of a lenticle. This can be called femtosecond lenticle extraction (FLEx). Subsequently, the lenticle can be removed with a pair of pincers after opening the flap. As a result, only one laser system is required, the use of an excimer laser can be forgone.
In some cases of refractive laser correction of eyes, it might become necessary, due to mistreatment or changes of the refractive condition, to perform, in turn, a laser-supported follow-up treatment. Thereby, the flap in the stromal bed must frequently be mechanically slightly detached; as a rule, however, at the perimeter of the flap, epithelium has grown in dependence of the elapsed time since the preceding treatment. In order to perform follow-up treatment with as few complications as possible, the original points of incision should be reutilized as precisely as possible.
However, a mechanical repreparation of closely spaced incisions poses the risk of inadvertently removing additional or less tissue in comparison to an initial incision.
In principle, the repreparation of the flap is problematic since biomechanical changes after the initial laser treatment, particularly regression processes, might have altered the position of the incisions with regard to the front of the cornea, which serves as reference. As a result, complications due to an imprecise repreparation are virtually unavoidable during follow-up treatment if the aforementioned influences cannot be neglected for other reasons (for example, when the follow-up treatment is performed shortly after the initial treatment).
Said problem does not only occur during the repreparation of the flap but, under certain circumstances, also in the case of the femtosecond lenticle extraction during the actual refractive follow-up treatment if previous laser incisions are to be reutilized. If, for example, such a lenticle extraction is performed minimally invasive, i.e., the lenticle is removed through a small peripheral incision (“Small Incision Lenticle Extraction;” SMILE) without opening an entire flap, it might become necessary during follow-up treatment to prepare a complete peripheral opening for a flap following said peripheral incision, thereby leaving a hinge at the edge of a previously untreated peripheral area.
A further option for improving defective vision through laser surgery is known from WO 2006/051364 A1 (20/10 Perfect Vision Optische Geraete GmbH). With this method, incisions are executed with a femtosecond laser in the stromal tissue in order to create a continuous cavity, particularly in cylindrical form, without ablation of tissue. During the collapse of the cavity due to the intraocular pressure, the cornea relaxes and takes on a new form with altered curvature. In principle, radial keratotomy (Fjodorov), and astigmatic keratotomy are similar methods. With the appropriate placement of relaxing incisions, which can also be performed through laser surgery, defective vision can be improved.
However, with said method it is difficult to immediately recognize an incomplete treatment, a mistreatment, or an undercorrection or overcorrection. By contrast, during the Femto-LASIK method, e.g., a non-detaching flap due to insufficient incisions is immediately recognized, allowing for the initiation of an immediate follow-up treatment without complications. Therefore, one or several follow-up treatments might also be required with the aforementioned methods in order to iteratively achieve an improvement of the defective vision, for example. However, in the meantime, the biomechanical changes, as described above, can occur.